1. Field of the Invention
The invention relates to data value determination. More particularly, the invention relates to determining the values of data retrieved from data storage devices such as holographic memory devices and other data storage media.
2. Description of the Related Art
Accurate retrieval or reproduction of stored data from data storage media has been important to the development of data storage systems such as holographic memory systems. Typically, a holographic memory or data storage system involves three-dimensional storage of holographic representations (i.e., holograms) of data members as a pattern of varying refractive index and/or absorption imprinted in a storage medium such as a crystal of lithium niobate. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
When stored data is to be recalled, detector arrays are used for reading the information emerging from the memory pages. Typically, the recalled holographic information is projected onto an imaging unit such as a charged coupled device (CCD) or a CMOS detector array such as an active pixel sensors (APS), which are sensitive to the emerging optical information. A decoding unit coupled to the imaging unit then determines the corresponding values of the data initially encoded and holographically stored.
However, reproduction of stored data in holographic storage devices is less than ideal due to effects often inherent in holographic memory systems. For example, the number of data members imprinted in the storage medium typically is inversely proportional to the diffraction efficiency and thus the reliability of the holograms stored therein to be reproduced upon read out. Other factors often affecting the quality of the stored data include time-dependent fluctuations in light intensity throughout the storage medium, physical and optical disturbance of the mechanical relationship between system components, thermal expansion and other temperature-induced variations throughout the system, and other noise introduced into the system.
Because of the often poor quality in retrieving or reproducing such data, the value of a data member often is represented by comparisons of multiple data members. For example, it is possible to employ some sort of differential encoding scheme, in which a particular data value sought to be retrieved results from the comparison of one data member to another. Alternatively, it is possible to use a reference encoding scheme, in which the value of a data member is compared to one or more known reference data member values. See, for example, U.S. Pat. No. 5,450,218 and U.S. Pat. No. 5,838,469 entitled "Apparatus And Method For Processing Data Stored In Page-Wise Memory", filed on even date herewith.
However, such encoding schemes require the use of more than one data member to represent the data value of a single data member. For example, in typical differential encoding schemes, a single data value is based on the value of a first data member relative to a second data member. In this manner, two data members are needed to represent a single data value or data bit. Therefore, roughly half of the data members stored and retrieved from the storage medium are not actual data values but are "overhead" necessary for implementation of the encoding scheme. As such, the theoretical maximum data storage efficiency for conventional differential encoding schemes approaches approximately 50%.
In conventional local reference encoding schemes, an individual data value is based on the value of a first data member relative to one or more reference data members. In this manner, for every n data values represented, reference encoding schemes typically require at least approximately n+1, n+2 or even more data members. Thus, the data storage efficiency approaches (n/(n+2))% or (n/(n+1))%. However, due to the inherent variations in holographic storage media discussed above, in practice n typically is kept small so that existing variations are somewhat consistent for both the reference data members and those data members being compared to the reference data members.
Therefore, although encoding schemes exist for improving data storage efficiencies among, e.g., holographic and other memory systems, a need still exists for reducing overhead among data members stored within and retrieved from data such storage media.